Adds Track-Only App Support (Addresses #119 and Sets Groundwork for #44) (#123)

- All Sources now have a "Track-Only" option that will prevent Obtainium from looking for APKs (though the App must still have a release of some kind so that a version string can be grabbed).
    - These Apps cannot be installed through Obtainium, but update notifications will still be sent.
    - The user needs to manually mark them as updated when appropriate.
    - This addresses issue #119.
    - It also partially addresses #44 by allowing some sources to be configured as "Track-Only"-only. The first such source (APKMirror) will be added later.
- Includes various UI changes to accommodate the above change.
- Also makes App loading a bit more responsive (sending Obtainium to the background then returning will now cause App re-load to pick up changes in App versioning that may have been made in the meantime, for instance through update checking).

lib/app_sources/fdroid.dart

@@ -48,9 +48,6 @@ class FDroid extends AppSource {
           .where((element) => element['versionName'] == latestVersion)
           .map((e) => '${apkUrlPrefix}_${e['versionCode']}.apk')
           .toList();
-      if (apkUrls.isEmpty) {
-        throw NoAPKError();
-      }
       return APKDetails(latestVersion, apkUrls);
     } else {
       throw NoReleasesError();